Every so often I read a book with footnotes, and I've seen them use Ibid. followed by page numbers - but I have no idea what this term means.

At first I thought it was a reference to a classical author (spurred on a little bit by the character of Ibid in a Discworld book), but I realise now that it must have some technical meaning for referencing sources.

There is a link between this term and the character in the Discworld, though it's the opposite relation than the one you assumed. Since ibid is such a commonly encountered Latin term, at least for people in academia, Pratchett named the character after that. See here: wiki.lspace.org/wiki/Ibid
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Avner Shahar-KashtanJul 26 '12 at 11:10

Ibid is a contraction of ibidem, a Latin word meaning “the same place.” This term is most commonly used for footnoting in scholarly texts, allowing the author to say “ibid” instead of citing a lengthy title. In legal texts, people may use “id,” a shortening of “idem,” a word which means “as mentioned previously.” If you've ever been reading a text and wondering about the identity of this “ibid” person who seems to get cited all the time, now you know!

Essentially, “ibid” is a fancy form of ditto marks. If, for example, you are referencing something like The Effects of Factory-Produced Emissions on the Greater Nile Watershed: An Environmental Study, that's a long title to have to refer to again and again. Instead, you can reference the title in a footnote, and then use “ibid” in future footnotes. If you move to a new location in the text, you can alert your readers with “Ibid (page 23)” or “Ibid, 23,” depending on what kind of citation format you are using.

When a new source is introduced, the “ibid” process begins all over again. In other words, if you cite The Effects of Factory-Produced Emissions on the Greater Nile Watershed: An Environmental Study once and follow with four additional citations marked with “ibid” before moving on to Cultural Practices in the Southern Nile Floodplain, an “ibid” after this source would refer to Cultural Practices in the Southern Nile Floodplain, not to the original text.

"ibid" is short for ibidem, Latin for "in the same place." It's an expression used in bibliographies when authors repeatedly cite the same source. So instead of typing out Sharks: Mighty Finned Killers of the Deep every time you refer to the book you used in your science project, you simply type "Ibid" for each reference after the first one, then cite the page number to which you're referring.